Generally, exhaust gas generated from engines contains plenty of harmful ingredients such as CO, HC, nitrogenous compound (NOx), etc. To reduce emission of pollutant gas such as NOx, exhaust gas recirculation (EGR) systems are typically used.
In such an EGR system, some exhaust gas, which is discharged after combustion, is supplied to and mixed with an intake gas mixture and then drawn into a combustion chamber. Thereby, the density of the gas mixture is reduced without changing the air-fuel ratio of the gas mixture, and thus the combustion temperature is reduced.
That is, in the EGR system, when there is a need for reducing emission of pollutant gases such as NOx, depending on conditions of the operation of the engine, some exhaust gas is added to intake gas through an EGR valve before being supplied along with the gas mixture into the combustion chamber. Here, exhaust gas which is drawn into the combustion chamber is inert gas, and the volume of intake gas does not change. Therefore, the density of gas mixture is relatively reduced, so that flame spreading speed is reduced, and thus combustion speed is also reduced. Hence, an increase in the combustion temperature is prevented. As a result, emission of pollutant gas such as NOx is reduced.
In particular, a low pressure exhaust gas recirculation (LP-EGR) system is configured such that a valve is provided on a rear end of a turbocharger so that exhaust gas can be supplied to a front end of a compressor of the turbocharger and recirculated, thus reducing emission of nitrogenous compounds (NOx).
To employ such an LP-EGR system, a differential pressure must be formed. For this, a back pressure valve is generally used to increase back pressure on a rear end of a diesel particulate filter (DPF), and thus form differential pressure. A DPF is a post-processing apparatus. The back pressure valve is configured such that the degree of opening of a flap is adjusted by an actuator provided in the valve, so as to control the magnitude of differential pressure in the rear end of the DPF.
In conventional techniques, the control constant of the back pressure valve is set to be comparatively large so that responsiveness of the valve is relatively high. However, if the back pressure valve is operated with a comparatively large control constant under conditions such as idling conditions in which noise of the engine is comparatively small, there is a problem in that a driver may feel discomfort because of noise generated from gears of the actuator.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that anything disclosed in the present disclosure is already known to those skilled in the art.